Control of contrast in diazotype materials



Patented Mar. 29, 1949 Carl Botlrin and Sam Charles slifkin, Binghamton, N. Y., assignors to General Aniline & Film Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application May 13, 1946, Serial No. 669,456

3 Ciaimns.

l 'Ihe present invention relates to diazotype light-sensitive materials and more particularly to the control of contrast in. diazotype reproductions. Diazotype reproductions or prints are generally obtained by exposing a photographic paper which contains a layer comprising a stabilized light-sensitive diazo compound and an azo component or coupler through a pattern such as a transparent negative or positive, a line drawing, printed matter and the like, and after exposure developing the image or print by treating the-exposed material with an alkali such as, for example, ammonia.

In the past, the literature dealing with diazotype processes has almost universally contended that due to the high contrast inherent in the method it was not possible to make good quality continuous tone reproductions. More recently, by improvements in formulation and by the use of proper bases such as baryta coated and/or gelatin surfaces, prints of good quality and normal contrast have been obtained. These, however, have been achieved at the expense of some loss in maximum densities.

It is accordingly an object of our invention to provide new light-sensitive diazotype compositions which permit control of contrast in diamtype reproductions. It is a further object of our invention to provide control of contrast while retaining maximum densities of the prints. It is a further object of our invention to provide control of contrast and increase in the printing latitude of diazotype materials.

According to our invention, the above objects are accomplished by incorporating in the diazotype coating compositions or solutions an ascorbic acid, such as ascorbic acid or iso-ascorbic acid. We have found that by the incorporation of these compounds in the light-sensitive diazo composition that a lower and controllable contrast of the diazo print can be obtained. As a result of our invention we achieve the production of diazotype papers of varying degrees of contrast ranging in gamma of from about 2 to below 1, of greater printing latitude, and retention of maximum densities in the print. We believe that the ascorbic and iso-ascorbic acid achieve the desired results described herein by selectively reducing or weakening the higher densities only without affecting the lighter tones or high-lights in the print. By employing a paper having a slower printing speed, that is, a paper coated with a diazotype solution of increased concentration, rich maximum density tones may be retained Without any loss in the high-light or low density values. This automatically makes for a paper of greater printing latitude. Thus, where in ordinary diazotype materials an over-exposure would completely or almost completely decompose the diazo compound in the half-tone or high-"light areas, thereby resulting in extremely high contrast :prints, by the employment of ascorbic and iso-ascorbic acid, the'tresultin'g print, on the other hand, exhibits normal contrast with rich maximum density tones.

The following examples will illustrate our invention but it is to be understood that the invention is not limited thereto.

Example 1 A coating solution was prepared by mixing the following ingredients in the proportion given at room temperature:

Water cc 8!) Glycol cc 5 Citric acid grams 5 Resorcinol d0 5 Ascorbic acid do 0.4 Saponin do 0.1 The zinc chloride double salt of p-diazo-N- dimethylaniline grams 2 A stock paper was coated with the above solution in the conventional manner employing a doctor blade to remove the excess solution and dried. Samples of these coated papers were then exposed to light under an original positive containing full density and half-tone areas, and the thus exposed sensitized paper was developed in the well-known manner by exposure to an alkali, such as, ammonia gas. The resulting print was of rich maximum density with good quality continuous tones, and without any loss in high-light or low density values.

On the other hand, where a print was made in the same manner except that ascorbic acid was omitted from the coating solution, the resulting print was highly contrasty and the high-lights stood out in sharp contrast to the higher density areas.

Example 2 The same composition was employed as in Example 1 except that for the ascorbic acid employed therein, there was substituted iso-ascorbic acid. The print obtained from a light-sensitive material coated with the solution was in all respects similar to the print obtained according to Example 1. It showed good continuous tones, was of high quality and exhibited normal contrasts.

' that is, whether an extreme flattening of the contrast or just a slight reduction is required. In the former case, a larger concentration of the ascorbic acid is employed, while in the latter case, a smaller concentration is utilized. In general, these contrast control agents are effective in low concentrations so that only a small amount need be employed as, for example, a range from about 0.2% to about 1%. However, any amount which will give the desired amount of control of contrast may be employed.

We do not know the exact mechanism by which ascorbic acid and iso-ascorbic acid control contrasts, but in their action they appear to selectively reduce or weaken the higher densities without affecting the lighter tones. We believe, although we do not wish to-be bound thereby, that this result flows from a combination of such factors as concentration and oxidation-reduction potential. Thus, Where enough undecomposed diazo is left in the paper, the ascorbic acids exert their influence and a reduced density results. By employing the diazo dye components in higher concentrations in the sensitizing solution, although reduced contrast results, rich maximum density tones are retained without any loss in high-light or low density values. As has been mentioned above, this automatically makes for increased printing latitude.

It is to be understood that the invention is not to be limited to the specific examples given above. We have found that the contrast of diazotype materials in general can be controlled in accordance with the present invention. Such diazotype sensitized materials as are prepared in accordance with the procedures given in U. S. Patent Nos. 1,444,469, 1,628,279, 1,803,906, 1,816,989 and 2,158,836 lend themselves to control of the contrast of the prints obtained therefrom by adding to the diazotype compositions described therein, the ascorbic acids herein mentioned.

We claim:

1. A light-sensitive diazotype material which comprises a diazo compound, a coupler, and an ascorbic acid.

2. A light-sensitive diazotype material which comprises a diazo compound, a coupler, and ascorbic acid.

3. A light-sensitive diazotype material which comprises a, diazo compound, a coupler, and isoascorbic acid.

CARL BOTKIN. SAM CHARLES SLIFKIN.

REFERENCES CITED UNITED STATES PATENTS Name Date Sprongerts Aug. 9, 1932 Number 

